4
Nitrogen
WATER--NITROGEN INTERPLAY
SETS YIELD POTENTIAL
A GRDC-funded project investigated the relationship
between the yield-driving factors of nitrogen and water
By Associate Professor Victor Sadras
NITROGEN PRACTICES NEED to
be continuously revised to make sure
new wheat varieties can achieve their
full yield and protein potential.
GRDC-supported research shows that
selecting for yield and grain protein in
winter-rainfall regions has significantly
increased the capacity of new wheat
varieties to absorb nitrogen from soil.
An example of this breeding advance
is the fact that improved nitrogen uptake
has helped to lift water use efficiency
(WUE) from 20 kilograms of nitrogen
uptake per hectare per millimetre of water
use in 1980s varieties to 25kg/ha/mm in
current varieties in southern Australia.
If a nitrogen deficiency is allowed
to occur, then a gap will open between
achieved yield and yield potential.
Preliminary estimates indicate 0.6kg/
ha/mm of nitrogen uptake will close the
yield gap, but grain protein influences
this benchmark. For example, closing the
gap may require 0.7kg nitrogen uptake
per ha per mm for grain with 14 per cent
protein, and 0.5kg nitrogen per ha per
mm for grain with 11 per cent protein.
Matching nitrogen inputs to crop
water availability and variety is a
balancing act that directly influences
productivity and profitability.
A scoping study led by the South
Australian Research and Development
Institute (SARDI), as part of GRDC's More
Profit from Crop Nutrition II, provided
insights on the interplay between water
and nitrogen from diverse perspectives.
These were captured by a team including
regional agronomists and scientists from
SARDI, CSIRO and the universities of
Adelaide and Queensland. The aim was
to identify gaps to guide future research,
development and extension activities.
WATER AND NITROGEN CAN
HAVE A COMPLEX RELATIONSHIP
¢ The effects of water and nitrogen on
crop yield can be cumulative. This
means if nitrogen supply increases
yield by 10 per cent, and water
supply increases yield by 10 per
cent, then the total combined
yield gain is 20 per cent.
¢ Or they can have a non-cumulative
effect. This is when the combined
effect of water and nitrogen can
be higher (synergy) or lower
than that expected from the sum
of the individual effects.
¢ Wa ter and nitrogen can be involved
in trade-offs. For example,
increasing the rate of nitrogen
improves water use efficiency but
reduces nitrogen use efficiency.
Understanding these relationships
between water and nitrogen is
important to capture synergies
and manage trade-offs.
The project looked at the role of
physiological, agronomic, economic
and breeding factors in the relationship
between water and nitrogen.
This agronomic component involved
local information from experienced
agronomists and data collected from 274
wheat crops grown in a range of soil,
climates and management systems.
These trials showed that:
¢ fertiliser rates of below 25kg nitrogen/ha
generated a median wheat yield of
2.7t/ha and median grain protein
concentration of 10.6 per cent;
¢ a rate of 150kg nitrogen/ha returned
median yield of 5.3t/ha and median
grain protein concentration of
13.1 per cent;
¢ yield and biomass increase in
response to fertiliser was two to three
times higher under high-yielding
conditions than under water stress;
¢ variation in yield was mostly related
to variation in biomass; and
¢ median harvest index was relatively
stable, ranging from 0.38 with less
than 25kg nitrogen/ha to 0.41 at
150kg nitrogen/ha. (This contrasts
with the perception that excess
nitrogen leads to low harvest index.)
This project also looked at nutrient
balance -- comparing nitrogen export by
grain with nitrogen fertiliser inputs.
A coarse "threshold" across soils,
climates and practices indicated that
fertiliser rates below 50kg nitrogen/
ha would mean that nitrogen export
exceeds nitrogen input so the crop is
essentially 'mining' the soil of nitrogen.
This supports a long-term experiment
in Victoria, where the amount of
nitrogen removed was the equivalent
of nitrogen fertiliser applied at rates
of between 40 and 80kg N/ha.
From a physiological perspective,
fertiliser drives a trade-off between water
use efficiency and nitrogen use efficiency
This means each additional unit of nitrogen
will increase yield per unit of water use,
and at the same time it will reduce yield
per unit of fertiliser -- irrespective of
crop, soil, climate and management.
It is impossible to maximise both
efficiencies at the same time, so any fertiliser
rate decision must involve a trade-off that
reflects individual environmental factors,
economic considerations and attitudes to risk.
The project also looked at
nitrogen management decisions under
uncertain seasonal conditions.
In rain-fed cropping systems, yield and
consequently profit depend not only on crop
nutrition, but also on matching genotype and
management to the growing environment
and expected seasonal conditions. □
G RDC Research Code DAS00157
More information: Victor Sadras,
victor.sadras@sa.gov.au
PHOTO: KELLIE PENFOLD
Fertiliser drives a trade-off between water
use efficiency and nitrogen use efficiency.
A broad ‘rule of thumb’ across
soils, climates and practices is
that fertiliser rates below 50kg
nitrogen per hectare lead
to more nitrogen being taken
from the soil than put in.
FAST FACT